A few things to consider...
If the change of ratio makes little difference - what is the timing on the motor? In a GT12 try around 40deg assuming that zero on the markings at the back really is zero.
Cells - how old are they? If they've seen a bit of action the internal resistance might have gone up to the point where they don't deliver the volts. If you can try someone else's cell with the same settings as when using your cell it will give you a clue.
Although you've been up and down the pinions, what size tyres are being used? Assuming that you are using 46mm diameter tyres (my best guess!) then...
On a 26T pinion that is 53.6mm/rev - too low. On a 35T pinion that is 72mm/rev - arguably a little high but if you have no timing on the motor it would explain why you saw little change in speed. On 40deg timing aim for 68mm/rev to start with.
The evidence you have given us points me to the motor timing, cells or the end-points issue already suggested above. Come back with more information and we'll help some more!
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